Vacuum cleaners are designed to separate dirt and dust from an airflow. Commonly, a motor and fan unit generates an airflow which draws dirt- and dust-laden air into the vacuum cleaner through a dirty air inlet. The airflow then passes through a form of separating apparatus such as a porous bag or a cyclonic separator (see, for example, EP 0 042 723) to remove dirt and dust from the airflow.
Irrespective of the type of separating apparatus used, there may be a risk of a small amount of dirt and dust passing through the separating apparatus and being carried to the motor and fan unit. It is undesirable for dirt and dust particles to pass through the fan of a motor and fan unit because the fan may become damaged or may operate less efficiently. In order to reduce this problem, some vacuum cleaners include a fine filter in an airflow path between the separating apparatus and the airflow generator (see, for example, GB 2 320 419). This filter is commonly known as a pre-motor filter and is used to extract any fine dirt and dust particles remaining in the airflow after it has passed through the separating apparatus.
It is also known to provide a filter in an airflow path downstream of the airflow generator in order to extract any remaining dirt and dust particles prior to the airflow exiting the appliance. This type of filter is known as a post-motor filter. The post-motor filter also captures particles produced by the brushes of the motor. The post-motor filter may be of the High Efficiency Particulate Air (HEPA) type, to remove very small particles from the airflow, such as smoke and allergens.
Filter assemblies for removing dust or debris from the air stream of a vacuum cleaner or other dust retaining appliances are therefore common. Such filter assemblies generally comprise at least one filter located in a filter housing.
During normal operation of a vacuum cleaner, dirt and dust may be deposited on a filter and, after a period of time, it could become blocked. Blockages reduce the efficiency at which a vacuum cleaner operates. Therefore, a typical filter will occasionally need to be replaced or cleaned in order to maintain the performance of the vacuum cleaner. In order to allow cleaning or replacement of the filter, it is common for such filters to be removable from a vacuum cleaner. Appliances with replaceable filters are common but the purchase of replacement filters can add to the overall cost of maintenance of the appliance.
Re-useable filters can reduce filter maintenance costs. U.S. Pat. No. 4,902,306, for example, discloses an air filter assembly including electrostatic filter elements and a foam filter that can be cleaned by washing.
Another type of known filter assembly is that used on the Dyson range of vacuum cleaners, for example, on model numbers DC04, DC07, DC12, DC14 and DC15. In these vacuum cleaners, the filter assembly essentially comprises a filter housing or filter cage, a first filter portion and a second filter portion. The filter housing has a round or rectangular cross-section and is manufactured from a suitable plastics material. The filter housing is adapted to receive the first and second filter portions. The second filter portion may be bonded to the filter housing to prevent misalignment. The first filter portion is usually washable. The second filter may also be washable and the second filter portion and the filter housing must be washed together if the second filter portion is bonded to the filter housing. After washing and after drying the filter assembly can be returned to the vacuum cleaner for further use.
The principle by which filter assemblies of this type operate is described in GB 2349105 and EP 1239760B.